The present invention focuses on a dedicated outdoor air treatment and ventilation system to deliver properly conditioned outdoor air in HVAC systems using terminal equipment such as fan coils, water source heat pumps and blower coils. The primary benefit of using this type of system is the ability to properly heat, cool and/or dehumidify the outdoor ventilation air independently of the other equipment in the system.
Poor indoor air quality can pose many risks for the building designer, owner and manager. The quality of the indoor environment can affect the health and productivity of the building occupants and even affect the integrity of the building structure itself. A building's indoor air quality is the result of the activities of a wide variety of individuals over the lifetime of a building, the atmosphere surrounding the building, the building materials themselves, and the way in which the building is maintained and operated. The interaction of these variables make achieving acceptable indoor air quality a complex, multi-faceted problem. Although complex, the fundamental factors which directly influence indoor air quality can be divided into four categories: (a) contaminant source control, (b) indoor relative humidity control, (c) proper ventilation, and (d) adequate filtration.
Ventilation is the process of introducing conditioned outside air into a building for the purpose of diluting contaminants generated within the spaces and of providing makeup air to replace air which is lost to building exhaust. The amount of ventilation air so required is established by building codes and industry standards, and varies with the intended use of the occupied spaces. Most building codes reference ASHRAE Standard 62-89 "Ventilation for Acceptable Indoor Air Quality" either in part or in entirety as a minimum requirement for ventilation system design. This standard is hereby incorporated by reference. ASHRAE Standard 62-89 recommends that "relative humidity in habitable spaces be maintained between 30 and 60 percent to minimize the growth of allergenic and pathogenic organisms". Additionally, indoor relative humidity levels above 60 percent promote the growth of mold and mildew, can trigger allergenic reactions in some people, and have an obvious effect on personal comfort. Extended periods of high humidity can damage furnishings and even damage the building structure itself. Controlling moisture levels within the building and the HVAC system is the most practical way to manage microbial growth.
The increased attention to indoor air quality (IAQ) is causing system designers to look more carefully at the ventilation and humidity control aspects of mechanical system designs particularly including dedicated outdoor air treatment and ventilation systems. These types of systems separate the outdoor air conditioning duties from the recirculated air conditioning duties. For simplicity, the present invention is discussed in terms of constant volume systems but is also intended to encompass variable air volume (VAV) systems.
Constant volume (CV) systems deliver a constant volume of airflow to a space at a temperature that varies in response to the thermal (or sensible load) requirements of the space. Examples of equipment commonly used in CV applications include direct expansion rooftop units, indoor air handlers, outdoor air handlers, and terminal products such as fan coils, unit ventilators, water source heat pumps, and blower coil units.
Constant volume systems are traditionally controlled based on space sensible temperature only. Any control of latent energy such as humidity is a byproduct of the sensible cooling process. Basic psychrometrics dictate that, to reduce space relative humidity, the supply air must be at a lower dewpoint than the space. At high space sensible loads, the leaving air temperature of the cooling coil is low, usually below the target dewpoint, resulting in adequate dehumidification. However, when the sensible load of the space is low (i.e., under part load conditions), the controller of the constant volume system responds by increasing the leaving air temperature to avoid overcooling the space. If the dewpoint of the air leaving the cooling coil is now above the targeted dewpoint for the space, inadequate dehumidification of the space occurs.
One approach to dealing with the reduced latent capacity of a constant volume system under these part load conditions is to separate the system outdoor and recirculated air paths. In such an arrangement, a dedicated central unit heats, cools and/or dehumidifies the outdoor air to an approximate comfortable temperature (65-80.degree. F.) and an approximate low dewpoint (42-53.degree. F.) dictated by the desires of the building owner or operator. Under most operating conditions, the outdoor air unit over cools the outdoor air to remove the required moisture and then reheats it back up to a room neutral condition of about 65-80.degree. F. to avoid over cooling the space and unnecessary reheating at the terminal unit. Often the energy to reheat the entering outdoor air is recovered energy from the cooling process such as condenser heat.
Prior art systems have not been optimized to control the sensible and latent cooling of a unit providing outside air. Additionally, the sizing of the heat exchange coils in such a unit has not been optimized.